meson: Provide our Python tools for builds

Several of our tools are installed and are used by other projects to generate code. However, there is no 'install' when projects use glib as a subproject.

We need some way for glib to 'provide' these tools so that when some project uses glib as a subproject, find_program('glib-mkenums') will transparently return the glib-mkenums we just built.

Starting from Meson 0.46, this can be done with the meson.override_find_program() function.

As a bonus, the Meson GNOME module will also use these 'overriden'/'provided' programs instead of looking for them in PATH.